Multi-ampere duopigatron ion source

ABSTRACT

A duoplasmatron ion source is modified to provide a large plasma surface with a uniform density at a target cathode. The target cathode and the acceleration and deceleration electrodes are gridded or multi-apertured and are spaced in close proximity each to the others with the apertures being in alignment. With such an arrangement, it is possible to extract multi-ampere bright ion beams at energies of tens of KeV. Conversion of the ion beam to a neutral particle beam can be readily accomplished by addition of a gas cell.

United States Patent 1 Morgan, Jr.

[ June 19, 1973 MULTI-AMPERE DUOPIGATRON ION SOURCE [75] Inventor: OraB. Morgan, Jr., Oak Ridge,

Tenn.

[73] Assignee: The United States of America, as represented by theUnited States Atomic Energy Commission.

[22] Filed: Apr. 13, 1972 [21] Appl. No; 243,676

[52] U.S. Cl. 250/419 SE, 313/63 [51] Int. Cl. H01] 37/08 [58] Field ofSearch 250/4l.9 SE, 41.9 SA,

250/4l.9 SB; 313/63 56] References Cited UNITED STATES PATENTS 3,238,4143/1966 Kelley 250/495 R MAGNET 19 SUPPLY 0-3A TARGET CATHODE 10o KV O-3AO-25KV O-OiSA Primary Examiner-James W. Lawrence Assistant ExaminerC. E.Church Attorney-Roland A. Anderson [57] ABSTRACT A duoplasmatron ionsource is modified to provide a large plasma surface with a uniformdensity at a target cathode. The target cathode and the acceleration anddeceleration electrodes are gridded or multi-apertured and are spaced inclose proximity each to the others with the apertures being inalignment. With such an arrangement, it is possible to extractmulti-ampere bright ion beams at energies of tens of KeV. Conversion ofthe ion beam to a neutral particle beam can be readily accomplished byaddition of a gas cell.

6 Claims, 2 Drawing Figures FILAMENT SUPPLY O-3OA ARC - SUPPLY O-SOAo-soov Patented June 19, 1973 3,740,554

2 Sheets-Sheet 1 A FILAMENT was; 0-3OA 30A ARC 3 SUPPLY 0-50 A o-soovPRIOR ART Patented June 19, 1973 3,740,554

2 Shwets-Sheet 2 20 FILAMENT SUPPLY O-3OA MAGNET SUPPLY ARC SUPPLY O-5OATARGET CATHODE 0-25KV 0-0.5 A

MULTI-AMPERE DUOPIGATRON ION SOURCE BACKGROUND OF THE INVENTION Thisinvention was made in the course of, or under, a contract with theUnited States Atomic Energy Commission.

In the past ten years there has been a continued increase in the desiredor required ion current from ion sources. This has resulted in acontinued redefinition of the term intense ion beam to refer to beamsfrom milli-amperes (mA) to hundreds of mA to As at the present time.

The plasma required for the extraction of an ampere ion beam can beobtained from any of several types of electron oscillating or are typeion sources. Some of the most obvious of these are the calutron, Penningdischarge (PIG), Lamb-lofgren, radio-frequency, and duoplasmatron. Therehave been many developments in all of these sources that indicate thatthey are capable of producing intense ion beams. However, mostapplications are small when compared to various modifications of theduoplasmatron. Some of the various modifications to the duoplasmatronthat have been developed at the Oak Ridge National Laboratory (ORNL) aredescribed in an article published in The Review of ScientificInstruments, Vol. 38, No. 4, pp. 467-480, April 1967, titled Technologyof-lntense DC lonBeams by Ora B. Morgan et al.

The original duoplasmatron is a very efficient, compact source of ions,and a description thereof was published in Tabellen Der ElektronenIonenphysik and Ubergikroskopie by M. Von Ardenne, (VEB Deutscher Verlagder Wissenschaften, Berlin, 1956). Also this ion source is described inU. S. Pat. No. 2,975,277, issued Mar. 14, 1961, to M. Von Ardenne. Itcan be operated with high gas efficiency and a plasma density at theanode of approximately ions/cm. The ions can have an outward directedvelocity greater than 10 eV which can result in a plasma flow equivalentto a current density of about lOOA/ cm'. This density exceeds that fromwhich ions can be extracted and, therefore, the plasma must expand as itleaves the anode aperture. A second problem appears when trying tooperate with the high DC are current required for intense DC beams. Theelectrodes tend to overheat and result in damage to the anode. Improvedcooling is also needed for the intermediate electrode. Solutions to thisheating problem were found at the ORNL by incorporating a copper anodeand a heavily cooled intermediate electrode in a manner as desribed inthe above-mentioned article in The Review of Scientific Instruments.

Adding a fourth electrode to the duoplasmatron ion source resulted in asystem that can be operated stably at low pressure. Such an ion sourceis illustrated in FIG. 1 of the present disclosure which will bedescribed hereinbelow. In this prior art source, the added electrode istermed the target cathode and is seen to be distinguished from theexisting electrodes, namely, the intermediate electrode, anode, andextraction electrode. This prior art ion source as well as othermodifications to the original duoplasmatron ion source are described inthe above-mentioned article. Most of these prior ion sources have beenutilized in various experimental fusion research devices at ORNL.However, the ion beams that can be extracted from these prior devicesare, in most instances, less than 1 ampere since the plasma density overa relatively large cross section is not as uniform or intense as desiredin each of these devices thus limiting the output therefrom.

There exists a need for an ion source that produces a very intense ionbeam output that can be used as an injector for experimentalthermonuclear fusion devices, such as the ORMAK device at ORNL, wherethe energy of ion beam is utilized in the technique of neutral injectionheating. The desired goal of such an injector is to produce severalamperes (equivalent) of highly collimated neutral particles in an energyrange of about 5 to 60 KeV, and a very important requirement is that theion source produce a plasma with a uniform density over a large crosssection. Another requirement is to incorporate an electrode structurethat is highly transparent to the beam and which prevents self-causedelectric field distortions. The present invention was conceived to meetthe above need for an improved ion source in a manner to be describedhereinbelow.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide an improved ion source for use as a means of neutral injectionheating in a thermonuclear fusion device, wherein amperes (equivalent)of highly collimated neutral particles are produced by the ion sourceand an associated gas cell.

The above object has been accomplished in the present invention bymodifications of the four-electrode duoplasmatron ion source,illustrated in FIG. 1 of the present disclosure, for producing a largeplasma surface. These modifications include a new shaping of the ionsource in the area where the intense discharge of arc takes place (thatis, the target cathode area), and in the area where the ions areaccelerated. The anode is also reshaped, the target cathode is providedwith a plurality of apertures and is positioned in close proximity tothe acceleration and deceleration electrodes which are also providedwith a plurality of apertures, and a field shaping auxiliary coil ispositioned between the apertured target cathode and the intermediateelectrode. The reshaped anode, target cathode, and the auxiliary coilprovide a long mean free path for the electrons and the combined effectis to shape the electric and magnetic fields to produce a large uniformdensity plasma surface at the target cathode grid. From this modifiedion source, l-ampere beams can be extracted in steady state operationwith ion energies of from 1.5 to 5 KeV. When this ion source is operatedin the pulsed mode with 0.1 to 0.2 second pulses and 10% duty cycle ation energies of 20 to 40 Kev, 4-ampere beams can be extracted from thesource. At 30 to 40 KeV, about 60 percent of the ion beam is within ahalf angular divergence of l.2 with no magnetic lens. Using a hydrogengas cell coupled to the outputs of the ion source, this system produces2.6 amperes (equivalent) of 17.5 and 35 Kev I-I particles within a halfangular divergence of 12 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 showsa cross-sectional view of a fourelectrode douplasmatron ion source ofthe prior art; and

FIG. 2 shows a cross-sectional view of the improved ion source of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The prior art, four-electrodedouplasmatron ion source illustrated in FIG. 1 includes a source coil 1connected to a magnet supply 9, an intermediate electrode 3 providedwith a copper jacket at its lower tapered end which is cooled by meansofa cooling line 8, a filament cathode 2 connected to a fillament supply10, a copper anode 4 separated from the jacket of the electrode 3 bymeans of an insulator 7, a target cathode separated from the anode 4 bymeans of another insulator 7, and a nickel extractor electrode 6. Theanode 4 and the target cathode 5 are also cooled by means of additionalcooling lines 8. An arc supply 11 is connected between the anode 4 andthe filament cathode 2. Feed gas is fed into the interior of theintermediate electrode 3 in a conventional manner, not shown, such asshown in the above-mentioned Von Ardenne patent. The device of FIG. 1 isused with a suitable vacuum chamber, not shown, and a magnetic lens, notshown, is mounted below the extractor electrode 6 in a conventionalmanner for focusing the output beam from this ion source. It should benoted that in the operation of this ion source, only about 100 mA of theoutput beam is available as H; ions through 4-cm diameter apertureslocated at 178 and 250 cm from the magnetic lens.

The ion source of the present invention, illustrated in FIG. 2 of thedrawings, was conceived to provide a substantially greater extractableion beam than is possible with the above prior art source in a manner tobe described hereinbelow.

The improved ion source of FIG. 2 includes a source coil 24 connected toa power supply 19, a filament cathode 23 connected to a power supply 20,an intermediate electrode 13, an anode l4, and a target cathode assemblyincluding an annular hollow electrode member 15, annular hollow members35 and 36 which has mounted therebetween an auxiliary field shaping coil25, annular hollow members 37 and 38 which has mounted therebetween acylindrical target cathode support member 26 which supports an annulargridded target cathode 27. Member 35 is affixed to member 15, member 37is affixed to member 36, and member 38 is supported by a portion of theion source housing as shown. The spacing between the tip of theintermediate electrode 13 and the gridded target cathode 27 is about 9inches, for example, to provide for a relatively long electronoscillation space in the source plasma region, 41. The ion source ofFIG. 2 further includes an arc supply 21 connected between the cathodefilament 23 and the anode 14, a gridded acceleration electrode 28connected to the negative side of a power supply 31 and being cooled bymeans of a cooling line 39, a gridded deceleration electrode 29connected to ground and being cooled by means of a cooling line 40, andan ion beam acceleration power supply 30 connected to the gridded targetcathode 27, and to the deceleration electrode 29. The electrodes 13, 14,and are separated by means of ceramic insulators 17, as shown, and theseelectrodes are cooled by means of cooling line 18. The electrodes 27,28, and 29 are separated by means of the epoxy insulators 47.

Hydrogen feed gas, for example, is fed by means, not shown, into theinterior of the intermediate electrode 13 in the same manner asdisclosed in the abovementioned patented Von Ardenne ion source. A powersupply, not shown, connected to the auxiliary coil 25, is of the samepolarity as the source coil 24. If the ion source is used with a lenscoil 32 the auxiliary coil opposes the fringe field of the lens coil 32to provide a near zero field at the surface of the gridded targetcathode 27. The ion beam 34 which is extractable through the griddedelectrodes 27, 28, and 29 from the ion source may be directed upon atarget 33 for measuring the ion output of the ion source. When theoutput beam 34 is utilized for injection heating in a thermonuclearreactor such as the ORNL experimental fusion device ORMAK, the outputbeam 34 may be directed through a hydrogen gas cell for converting themolecular ions in the beam 34 to energetic neutral particles. Theseneutral particles are then injected into the interior of the ORMAKdevice in a manner as described in the AEC Report ORNL-TM-3472, issuedJuly 30, 197l. A description of the ORMAK facility without the ionsource of the present invention has been published in IEEE Trans.Nuclear Science NS-I8, No. 4.

The gridded electrodes 27, 28, and 29 are 5 -cm in diameter and eachelectrode is provided with 109 apertures of 3.75 mm diameter, forexample, with the electrodes being about 50 percent transparent. Theextraction gaps between the respective electrodes 27, 28, and 29, may befrom 3 to 6 mm, for example. The reshaped anode 14, target cathode 27,and the auxiliary coil 25 shape the electric and magnetic fields toprovide a large uniform density plasma surface at the grid of the targetcathode 27. The other two grids, referred to in FIG. 2 as theacceleration (reverse biased) electrode 28 and the deceleration(grounded) electrode 29, replace what would be the extractor electrodein FIG. 1. The radius of the apertures, r, of the electrodes 27, 28, and29 and the spacings of the electrodes, Z, are chosen for the desired ionenergy. The minimum spacing is dietated by the maximum stable electricfield gradient. The electrode apertures are assumed to act as adiverging lens. Therefore, the system is designed with an aspect ratio,R 2r/Z, that should yield a beam with a small divergence.

The system of FIG. 2 can be operated in steady state (continuous)operation and l-ampere beams can be ex tracted with ion energies of from1.5 to 5 KeV. However, for 0.1 to 0.2 second pulses and 10 percent dutycycle, 4-ampere beams can be extracted at ion energies of 20 to 40 KeV.At 30 to 40 KeV, about percent of the ion beam is within a half angulardivergence of 1.2' with no magnetic lens. Using a hydrogen gas cellconnected to the output of the ion source of FIG. 2, such a systemproduces 2.6 amperes (equivalent) of 17.5 and 35 KeV H particles withina half angular divergence of l.2 without the magnetic lens 32. Thedevice of FIG. 2 can be operated with or without the magnetic lens 32.Eliminating the magnetic lens makes the system simpler and makes itpossible to utilize all of the ion species. The device of FIG. 2 will beused for injecting through a gas cell into the ORMAK without a magneticlens as illustrated in FIG. 4 of the abovementioned AEC Report,ORNL-TM-3472, issued July 30, I971. The gas cell is a simple conductancelimited conical tube with the 1.2 angle divergence acceptable for ORMAK.

The improved and enlarged plasma surface obtained from the operation ofthe ion source of the present invention has provided so superior an ionbeam, as compared to prior ion sources, that the new term duopigatronhas been given to it. Among its advantages are that it is simple,compact, efficient, and operates stably over a large variation in sourcepressure. Many essential features of the present ion source such asspacecharge neutralization throughout the beam drift region, highelectric fields, elimination of regions where electric and magneticfields will cause PIG discharges, and use of either no lens or amagnetic lens are indicative that the ion source incorporates many ofthe outstanding ion source advances of recent years.

It should be understood that the 5-cm diameter size electrodes, for theelectrodes 27, 28, and 29, are not limited to this size or to a 50percent transparency, but may be scaled to even larger sizes when highercurrent exaction power supplies become available resulting in acorresponding increase in output currents from the ion source. Also, thenumber of apertures in the electrodes 27, 28, and 29 may range from tensto hundreds with the apertures having diameters from 2 to 5 mm. Thenumber of apertures and their sizes depend upon the use and currentoutput desired of the ion source.

It should be understood that other types of extraction geometries can beutilized with the ion source of the present invention. For example,multi-slits instead of multi-apertures can be used in the electrodes 27,28, and 29. Such a system would have the possibility of better thermalconductivity with the same transparency and this could make it possibleto increase the duty cycle.

The above-described ion source is readily applicable for use as theplasma source in the technique of neutral injection not only fortoroidal fusion devices, such as ORMAK, but also for mirror-type fusiondevices. Neutral injection is probably the most powerful and universallyapplicable technique for producing plasmas in any magnetic configurationat the high ion temperatures and densities necessary for abundant fusionreactions to take place. The present ion source provides larger ioncurrent at high ion temperatures than were previously available.

This invention has been described by way of illustration rather thanlimitation and it should be apparent that it is equally applicable infields other than those described.

What is claimed is:

'1. A high current duopigatron ion source comprising a heated filamentcathode; an intermediate electrode provided with an apertured, taperedlower end portion, said intermediate electrode enclosing said filamentcathode and adapted to receive a feed gas thereinto; a source coilencompassing the upper portion of said intermediate electrode; a sourcemagnet supply connected to said coil; a copper anode insulatingly spacedfrom said intermediate electrode, said anode being provided with acentrally disposed aperture and a hollow elongated tail portioncontingent with said aperture and extending longitudinally away fromsaid intermediate electrode; a target cathode assembly defining anelongated ion beam drift space and including a first, centrallyapertured electrode insulatingly spaced from said anode and encompassinga portion of said anode tail portion, a first tubular elongated memberaffixed to said first electrode, an annular auxiliary, field shapingcoil affixed to said first member, a second tubular member affixed tosaid auxiliary coil, a third tubular member affixed to said secondtubular member, a cylindrical target cathode support member affixed tosaid third tubular member, a fourth elongated tubular member affixed tosaid cathode support member and to the housing of said ion source, and amulti-apertured target cathode supported by said cathode support member;means for cooling said intermediate electrode, anode, and firstelectrode, a source of arc supply connected between said filamentcathode and said anode; a multiapertured acceleration electrode mountedbeyond and closely spaced from said apertured target cathode; a sourceof negative acceleration voltage connected to said accelerationelectrode; a source of high voltage connected to said target cathode;means for cooling said acceleration electrode; a multi-apertureddeceleration electrode connected to ground and mounted beyond and inclose proximity to said apertured acceleration electrode; and means forcooling said deceleration electrode, said anode and target cathodeassembly providing a long mean free path for the electrons produced bythe ion source discharge with the combined effect of shaping theelectric and magnetic fields to produce a large uniform density plasmasurface at the surface of said apertured target cathode therebypermitting the extraction of a high density and high temperature ionbeam through said apertured electrodes.

2. The ion source set forth in claim 1, and further including an annularlens coil mounted beyond said apertured deceleration electrode andencompassing said extracted ion beam for focusing thereof.

3. The ion source set forth in claim 1, wherein said multi-aperturedtarget cathode, acceleration electrode and deceleration electrode areeach S-cm in diameter and each is provided with 109 apertures of 3.75 mmdiameter, the respective spacing between said apertured electrodes beingfrom 3 to 6 mm.

4. The ion source set forth in claim 2, wherein said ion source isoperated in the continuous mode to produce an output ion beam of atleast 1 ampere with ion energies from L5 to 5 Kev.

5. The ion source set forth in claim 3, wherein said ion source isoperated in the pulsed mode with 0.1 to 0.2 pulses and 10 percent dutycycle to produce an output ion beam of at least 4 amperes at ionenergies of 20 to 40 KeV.

6. The ion source set forth in claim 5, and further including a hydrogengas cell connected to the output of said ion source to thereby produce2.6 amperes (equivalent) of 17.5 and 35 KeV H particles within a half angular divergence of 1.2".

1. A high current duopigatron ion source comprising a heated filamentcathode; an intermediate electrode provided with an apertured, taperedlower end portion, said intermediate electrode enclosing said filamentcathode and adapted to receive a feed gas thereinto; a source coilencompassing the upper portion of said intermediate electrode; a sourcemagnet supply connected to said coil; a copper anode insulatingly spacedfrom said intermediate electrode, said anode being provided with acentrally disposed aperture and a hollow elongated tail portioncontingent with said aperture and extending longitudinally away fromsaid intermediate electrode; a target cathode assembly defining anelongated ion beam drift space and including a first, centrallyapertured electrode insulatingly spaced from said anode and encompassinga portion of said anode tail portion, a first tubular elongated memberaffixed to said first electrode, an annular auxiliary, field shapingcoil affixed to said first member, a second tubular member affixed tosaid auxiliary coil, a third tubular member affixed to said secondtubular member, a cylindrical target cathode support member affixed tosaid third tubular member, a fourth elongated tubular member affixed tosaid cathode support member and to the housing of said ion source, and amultiapertured target cathode supported by said cathode support member;means for cooling said intermediate electrode, anode, and firstelectrode, a source of arc supply connected between said filamentcathode and said anode; a multi-apertured acceleration electrode mountedbeyond and closely spaced from said apertured target cathode; a sourceof negative acceleration voltage connected to said accelerationelectrode; a source of high voltage connected to said target cathode;means for cooling said acceleration electrode; a multi-apertureddeceleration electrode connected to ground and mounted beyond and inclose proximity to said apertured acceleration electrode; and means forcooling said deceleration electrode, said anode and target cathodeassembly providing a long mean free path for the electrons produced bythe ion source discharge with the combined effect of shaping theelectric and magnetic fields to produce a large uniform density plasmasurface at the surface of said apertured target cathode therebypermitting the extraction of a high density and high temperature ionbeam through said apertured electrodes.
 2. The ion source set forth inclaim 1, and further including an annular lens coil mounted beyond saidapertured deceleration electrode and encompassing said extracted ionbeam for focusing thereof.
 3. The ion source set forth in claim 1,wherein said multi-apertured target cathoDe, acceleration electrode anddeceleration electrode are each 5-cm in diameter and each is providedwith 109 apertures of 3.75 mm diameter, the respective spacing betweensaid apertured electrodes being from 3 to 6 mm.
 4. The ion source setforth in claim 2, wherein said ion source is operated in the continuousmode to produce an output ion beam of at least 1 ampere with ionenergies from 1.5 to 5 KeV.
 5. The ion source set forth in claim 3,wherein said ion source is operated in the pulsed mode with 0.1 to 0.2pulses and 10 percent duty cycle to produce an output ion beam of atleast 4 amperes at ion energies of 20 to 40 KeV.
 6. The ion source setforth in claim 5, and further including a hydrogen gas cell connected tothe output of said ion source to thereby produce 2.6 amperes(equivalent) of 17.5 and 35 KeV H* particles within a half angulardivergence of 1.2*.